How to add Multires subdivide button to my own script
Pass parameters to layout.operator

The modifiers property panel is in "button context" and has a member context.modifier which the associated operators use.  In the 3d view toolbox there is no such context member, but we can pass the modifiers name to the operator.

Also rather than row after row, can use column layout.

import bpy

class CustomPanel(bpy.types.Panel):
    """A Custom Panel in the Viewport Toolbar"""
    bl_label = "MultiModifer"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'TOOLS' 
    bl_category = "Sculpt"
    bl_category = "Tools"


    def draw(self, context):
        layout = self.layout
        obj = context.object
        mro = [m for m in obj.modifiers if m.type == 'MULTIRES']
        col = layout.column()
        for m in mro:                
            col.label(m.name)
            op = col.operator("object.multires_subdivide", text="Subdivide")
            op.modifier = m.name
            op = col.operator("object.multires_higher_levels_delete", text="Delete Higher")
            op.modifier = m.name
            col.label(text="Multires Levels")
            col.prop(m, "levels", text="Sculpt")

def register():
    bpy.utils.register_class(CustomPanel)

def unregister():
    bpy.utils.unregister_class(CustomPanel)

if __name__ == "__main__":
    register()